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Environmental Studies


The Nonliving Environment

Environmental studies also takes into account the nonliving components of the Earth’s environment, from the geology of the Earth itself to chemical and meteorological cycles taking place on the Earth’s surface and in its atmosphere.


The Earth and Its Surroundings

The Earth formed 4.5 billion years ago. Since then, it has developed and modified four main physical environments that interact strongly with one another.

Atmosphere: The layer of gases surrounding the Earth. The atmosphere is divided into several levels:

  • Mesosphere: The outermost layer of the atmosphere, extending from about 50–85km above the Earth’s surface.

  • Stratosphere: The middle level of the atmosphere, extending from about 13–50km above the Earth’s surface.

  • Troposphere: The lowest level of the atmosphere, extending from the Earth’s surface to approximately10–13km above the surface.

Hydrosphere: The cumulative water supply in and on the Earth, including liquid water, frozen water, and water in gaseous form.

Biosphere: The approximately 12-mile-thick area of the Earth in which life exists, on and around its surface. The biosphere includes parts of the atmosphere, hydrosphere, and lithosphere.

Lithosphere (geosphere): The solid portion of the Earth, which contains several layers:

  • Crust: The outermost layer of the Earth, which makes up only about 2% of the Earth’s volume. The crust is composed of eight principal elements: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. It is the only part of the lithosphere that humans have explored.

  • Mantle: The largest layer of the lithosphere, located beneath the crust. The mantle is composed of a mix of solid and liquid metal and nonmetal elements, such as oxygen, iron, magnesium, and silicon.

  • Core: The innermost central layer of the Earth. The core is composed of metals—mainly iron and some nickel—at very high temperatures.


Important Cycles in the Environment

A number of important biogeochemical cycles are constantly at work in the biosphere, recirculating nutrients and other elements through both the biotic and abiotic portions of ecosystems.

Carbon cycle: Carbon is the primary element of life and is found in all living organisms. The carbon cycle describes the movement of carbon through the environment.

  • Carbon is found in the atmosphere (primarily as CO2) and is also found dissolved in water.

  • Producers convert carbon dioxide, through photosynthesis, to organic (carbon-containing compounds) that store energy.

  • Consumers and decomposers use these carbon compounds to produce energy, breaking them down through respiration, thereby returning them to the atmosphere or to the water in the form of carbon dioxide.

  • Carbon also can be stored in organic material, such as trees or fossil fuels.

    • The process of taking carbon from the environment and storing it in another form is known as carbon sequestration.

    • Fossil fuels, such as coal, oil, and natural gas, are partially broken-down plant or animal tissues that have been stored and transformed in the Earth’s surface for long periods of time under high levels of heat or pressure.

  • Carbon can then be released from the above stored forms through combustion, or burning.

Nitrogen cycle: Nitrogen is an important nutrient commonly found as a gas in the atmosphere.

  • Because plants and animals cannot use the gaseous form of nitrogen easily, it must first be converted to other usable forms by bacteria through nitrogen fixation.

  • Some plants, called legumes, have nitrogen-fixing bacteria known as rhizobium in their roots. These plants take up the converted nitrogen and use it to form organic compounds, and then animals obtain these compounds by consuming the plants.

  • After the plant or animal tissue dies or is discarded, the remaining nitrogen, usually in simple forms, is converted back into atmospheric form by specialized bacteria.

Phosphorus cycle: Phosphorus is a crucial element required for energy transfer in organisms, but much of the Earth’s phosphorus is contained in rocks.

  • When rocks containing phosphorous break down because of erosion or other factors, plants then take up this phosphorus directly, and animals consume the plants.

  • Dissolved phosphorus and phosphorus waste eventually settles back to the ocean floor to become rock again.

Sulfur cycle: Another mineral, sulfur, is cycled in a manner similar to phosphorus.

Hydrologic cycle (water cycle): Water, the most vital requirement for most organisms, cycles through liquid, solid, and gaseous states throughout the Earth.

  • Water evaporates (changes from liquid to gas) from surface water on the Earth and is taken into the atmosphere.

  • It then condenses (changes from gas to liquid or solid) and falls down as precipitation (rain, snow, or ice) to the Earth’s surface.

  • Some of this precipitated water ends up as runoff (moving from the land back into water bodies), while some is taken up by plants.

  • Plants may either transpire the water (pass it out through their pores back into the atmosphere), or the plants may die or be consumed, passing that water onto higher level organisms, detritovores, or back onto land.

  • In addition to being stored in surface water, much of the Earth’s water exists as groundwater, below the Earth’s surface.